Separating membranes can, in principle, be present in the form of porous or non-porous layers. Among separating membranes, a distinction is made between homogeneous and asymmetric membranes. Homogeneous membranes are generally very thin to have the highest permeability per unit surface area possible. However, the mechanical stability of these thin membranes is frequently not sufficient. In asymmetric membranes, in contrast, a very thin separating layer is disposed on a mechanically stable porous substructure having high permeability.
Separating processes can be classified according to the phases involved (solid, liquid, gas) and are widely used in industrial applications. Membrane processes are also increasingly considered an alternative to conventional separation methods, such as distillation, crystallization, cryogenic separation, adsorption or extraction. However, use is disadvantageously limited by a lack of suitable membranes that not only have the necessary separation properties, but can also withstand harsh operating conditions (high temperatures, high pressures, water vapor, corrosive gases, corrosive liquids).
Membranes used commercially in separation processes to date generally comprise a polymer material, which has certain disadvantages, in terms of lacking mechanical, thermal and chemical stability, and can therefore only be used on a limited scale.
On the other hand, ceramic microporous membranes are already known, which comprise a functional layer made of zeolite, silica (SiO2) or carbon, and so-called hybrid SiO2 membranes which, in addition to SiO2, also comprise carbon atoms. These membranes are proposed as an alternative to polymer membranes. They can be used in a broader range of applications and moreover have a considerably longer service life. They are frequently resistant to the majority of organic solvents and are generally temperature-stable up to approximately 300° C. in the case of hybrid membranes, and possibly up to even higher temperatures.
In particular, zeolite membranes are suitable, both on a test scale and on a large scale, for use with separation processes of solvents, such as in the liquid phase (pervaporation (PV)) or in the vapor phase (vapor permeation (VP)). Some of these membranes have an extremely narrow pore size distribution around approximately 0.5 nm and are in particular suitable for separating water from all kinds of organic solvents.
Microporous amorphous SiO2 membranes have been known for more than 20 years. These membranes generally comprise a microporous SiO2 membrane layer having a pore size of greater than 0.5 nm, which is applied onto a mesoporous carrier comprising γ-Al2O3 by way of a sol gel process. Since their appearance, interest in these membranes has grown steadily, as they have sufficiently good separation properties for both water separation and gas separation.
One of the characteristic properties of such microporous ceramic gas separating membranes is thermally activated gas transport. It was found that, for high-quality membranes, the gas flow (flux) J through the microporous material increases as a function of the temperature according to the Arrhenius equation:J=J0 exp(−Eact/RT)where J=flux(mol m−2 s−1), J0=temperature-independent coefficient (mol m−2 s−1), R=gas constant (J mol−1 K−1), T=temperature (K), and Eact=activation energy (kJ mol−1).
Ascertaining activation energies for the gas transport in zeolite separating membranes and amorphous SiO2 separating membranes has shown that a relationship exists between the activation energy for the diffusion and the pore size and quality of the separating membrane. In general, the activation energy is greater with small pores than with large pores. Furthermore, a given separation membrane, the activation energy rises for with the quality thereof. A membrane of high quality is considered to be a membrane having no cracks or other defects (such as large pores) and having a narrow pore size distribution. For high-quality microporous ceramic membranes, the Eact value for the diffusing gases, such as He or H2, is generally higher than 10 kJ mol−1.
Additionally, it was found that separating membranes having a high activation energy for diffusion have molecular sieve properties for the smallest gas molecules, such as He or H2, which is to say the permeation increases with decreasing size of the gas molecules, or N2<CO2<H2<He. As a result, such separating membranes generally have a high gas selectivity for the smallest gas molecules (He, H2).
However, both the zeolite membranes and the amorphous SiO2-based membranes, which were contemplated as possible alternatives to polymer membranes, have the disadvantage that they are only resistant to acid and alkaline solutions to a limited degree. Moreover, it has been found that a number of zeolite membranes, and in particular traditional amorphous SiO2 membranes, do not have long-term stability with respect to hydrothermal conditions.
In practice, the use of amorphous SiO2 membranes is generally limited to dry applications, due to the particular sensitivity of the material to water. So as to improve material stability, a variety of modified SiO2 membranes were produced. These membranes were likewise applied by way of a sol gel process onto a mesoporous carrier comprising γ-Al2O3.
In addition to amorphous SiO2, these also comprised oxides, for example, such as ZrO2 or TiO2, or metals, such as Ni or Co, as a second component. However, this also did not yield suitable membranes having sufficient stability and a suitable pore size distribution for water or gas separation.
Hybrid carbon-containing SiO2 membranes are now also known, which have improved resistance to liquid water and water vapor at temperatures up to 150° C. (Hybsi®). Due to the partial SiO2 nature of these membranes, however, the scope of applications in water is likewise limited in practice. Moreover, thermal stability is limited to a maximum of 300° C., which restricts application options in gas separation. Such membranes are generally also not able to separate the smallest gas molecules (He, H2) from other gas molecules (such as CO2, N2, CH4) by way of molecular sieve processes since the pore size typically exceeds 0.5 nm.
Recently, graphene and graphene oxide have commanded enormous attention as potential membrane materials. Graphene and graphene oxide can be considered to be membranes that, strictly speaking, have a design that is only one atom layer thick. As a result, they constitute the absolutely thinnest artificially produced membranes.
Graphene is understood to mean a 2-dimensional carbon monolayer made of sp2 hybridized carbon, in which the carbon atoms are disposed in a honeycomb structure. Graphene oxide represents an accordingly functionalized form of graphene, in which oxygen-containing groups, such as hydroxyl, epoxy, carbonyl, carboxyl, lactone and quinone, are bound both on the edges and in the plane. Graphene oxide thus comprises both sp2 and sp3 hybridized carbon.
It was suspected that graphene and graphene oxide layers would not be permeable for both liquids and for gases, including helium, the smallest gas molecule. So as to arrive at a permeable membrane, it was therefore proposed to provide such a membrane with small apertures, which can be created by etching processes, for example.
As early as 2013, Lockheed Martin introduced a perforated membrane, Perforene™, which is only one atom layer thick, made of a graphene layer and has holes having a diameter of approximately 1 nm. The following advantages are cited for this membrane:
a) it is resistant to high pH values and corrosive cleaning agents;
b) it can be used at high temperatures;
c) it exhibits good separating action;
d) it has improved water flow and is therefore energy-saving; and
e) it is electrically conductive and hydrophobic, resulting in a reduced tendency toward clogging in real applications.
However, to date, no methods are known for exactly controlling pore formation. Other technological challenges include, for example, producing such membranes on a large scale and the robustness thereof.
Alternatively, it was also already proposed to stack several graphene or graphene oxide layers so as to arrive at what is known as few-layer graphene or graphene oxide. If the layers are stacked compactly and densely, lateral nanochannels form, in which small molecules, such as He, H2 or water, are able to pass, but which are impervious to larger molecules. Normally, and within the scope of the present invention, aggregates composed of fewer than 10 graphene or graphene oxide layers are referred to as few-layer graphene or graphene oxide. Those comprising more layers could generally be referred to as graphite oxide layers or graphite layers.
Furthermore, membranes have been described over the last 2 to 3 years which comprise stacked graphene oxide layers and are frequently referred to as graphene oxide membranes, and which underscore the general suitability thereof as separating membranes. Such membranes have a structure comparable to graphite, although the distance between the individual layers is larger, for example due to the bound functional oxygen-containing groups and intercalation of water molecules. In contrast with graphene layers, the exact structure and composition of the individual graphene oxide layers is not clearly defined. In contrast with graphene, graphene oxide comprises sp2 and sp3 carbon atoms, and for this reason the layers have a non-planar, irregularly undulated structure.
Graphene oxide can be obtained from graphite oxide, for example. Graphite oxide is understood to mean a non-stoichiometric compound made of carbon, oxygen and hydrogen, the empirical formula of which varies greatly depending on the production conditions. Graphite oxide, in turn, can be obtained from graphite using strong oxidizing agents, for example, by way of the known synthesis methods of Brodie, Hummer or Staudenmaier. During the production of graphite oxide, the graphene layers in the graphite are oxygenated, yielding a hydrophilic material. A particular property of the graphite oxide is that it can be colloidally dispersed in water, resulting in the formation of colloidal dispersions. The detachment of individual layers of the graphite oxide, for example by way of ultrasonic energy, results in monolayer graphene oxide.
The typical production route for the aforementioned kind of membranes provides for the application of individual graphene oxide layers onto a carrier material by way of vacuum filtration methods. During coating, a colloidal dispersion composed of monolayer graphene oxide and water is used.
Even though the monolayer or multilayer graphene oxide membranes already produced on a test scale exhibit very advantageous properties, they are presently not yet of sufficient interest for commercial fields of applications since they still involve a number of drawbacks. These include:
a) The membranes have not yet been provided with a suitable carrier that is able to withstand the harsh operating conditions in industrial applications (high temperatures, hydrothermal conditions, corrosive gases, corrosive liquids).
b) The membranes are generally not fixedly joined to the carrier.
c) Due to the lack of a robust carrier, the membranes are not pressure-stable.
d) The membranes are presently produced by way of methods that cannot be freely translated to a large scale.
e) The membranes presently do not yet have the size and dimensions needed in industrial plants.
f) The membranes do not have a molecular sieve effect, such as the known amorphous SiO2-based microporous membranes.
In the microporous ceramic membranes known from the literature (such as amorphous SiO2, hybrid carbon-containing SiO2, carbon), the mesoporous layer generally comprises γ-aluminum oxide (γ-Al2O3), titanium dioxide (TiO2), zirconium dioxide (ZrO2), silicon oxide (SiO2) or mixtures of the aforementioned materials, which can be produced by way of a sol gel coating process, for example. These layers, however, generally exhibit only limited chemical (γ-Al2O3, SiO2), thermal (TiO2, ZrO2) or hydrothermal (all) stability.
Furthermore, previously known graphene oxide membranes are produced at room temperature or treated at only slightly elevated temperatures up to 220° C. Due to the previously used carrier materials and/or the properties of the graphene oxide membranes, such as the layer thickness, it was not heretofore possible to use higher temperatures (T>220° C.) for such membranes.
Theoretically, a membrane composed of only few graphene layers appears to be particularly well-suited for the separation of smaller molecules (He, H2) from a gas mixture (such as with CO2, N2, CO or CH4), since the distance between graphene layers is in the order of magnitude of 0.335 nm. It is assumed that the distance between graphene oxide monolayers in few-layer graphene oxide and graphite oxide is greater than that between graphene layers. The literature reports distances between graphene oxide monolayers of 0.6 nm to more than 1 nm, as a function of the presence of water (liquid, vapor).